UK politics question

I gave up entirely on television news sometime in late 2001, but lately I've started watching it again — in the unlikely form (for an American) of Newsnight.

Thing is, the BBC produces Newsnight for domestic consumption. Which means sometimes I have absolutely no idea what they're talking about. So I figured I'd ask my friends across the pond to clue me in.....

In the year-in-review segment of Friday's show, tracing the year's poll fluctuations, they attribute Labour topping the Tories around June to Blair's departure. I get that: last time I was in England, I saw "Bliar" signs everywhere I went, and it was clear his days were numbered1. What I don't get is their explanation of the Tories' resurgence in October, which consisted of playing a clip of the shadow chancellor giving a speech in which he said "The next conservative government will raise the inheritance tax threshold to one million pounds." At which point the graph showed by far the sharpest change of the year, with the Tories' line jumping from about 32 to 40, crossing back above Labour's line.2 I'd appreciate it if someone could explain how a speech about inheritance tax could have such a huge impact.1 I saw a lot of signs making the Blair-liar pun, but this was by far the most common. I think it may also be the most effective political sign I've ever seen.2My American readers, if any are still with me at this point, should note that there are three significant parties in the UK, though the third hasn't held power for nearly a century.